HDV and AVCHD both have the same bitrate as DV - roughly 4 MB/s. The hard drive isn't the bottleneck with real time playback of those formats. A single drive, even at it's worst of 40 MB/s or so, is more than fast enough for multiple streams.
The CPU is the bottleneck here.
I would suggest you run our PPBM5 Benchmark and see how you compare to 224 other systems (February 11, 2011 total). Unfortunately there is no other Q9650 based system in the results, but we can always find something similar for comparison purposes. Until you test your system it is hard to tell where and if your system can be improved.
Also I guess there in no other CPU to upgrade your LGA 775 socket motherboard.
Rather than investing in 2 TB disks, continue saving and upgrade your whole system. A Q9650 is around 6 - 8 times slower than a fast system and it your main bottleneck in using AVCHD.